This invention relates to an optical pressure sensor comprising an optical fibre or waveguide with a well defined end part, and a casing having a cylindrical cavity with essentially the same transversal dimensions as the optical fibre, the end of the optical fibre being positioned at a chosen axial position inside the cavity thus to close one end of the opening.
Today there are a number of different types of pressure sensors based on different types of membranes which can be bent and read using electrical conductivity, capacitance or optical measuring of the distance between a part of the membrane or an element connected thereto, and an optical fibre.
There are a plurality of techniques for reading this types of distances, using white-light interferometry which may read the distance without ambiguity, but with moderate resolution. It may be measured with interferometric techniques having high coherence which provides large sensitivity for changes in the distance, but has an inherent ambiguity in the measurement. These techniques may be combined, e.g. by simultaneous use of coherent light with different wavelengths.
The measuring techniques are common knowledge, and may even to a certain extent be bought as commercially available instruments.
The optical measuring techniques has the following advantages:
Potentially good resolution.
Unaffected by electromagnetic influences, such as electromagnetic pulses etc.
Secure against electrical discharges/igniters, which make them useful in medical applications, and in uses related to explosives and inflammable environments.
Potentially very compact, especially in relation to light guides.
Membrane based optical sensors are usually expensive and relatively large. Mounting of a membrane against an optical fibre requires precision and, as the membrane responds to all movements the surrounding construction must be very rigid, while the membrane must be very deformable, especially to low pressures. This type of solution is usually expensive, partially because of the costs related to the probe production. The present invention has a potential of being an inexpensive and also very compact solution.
Swedish patent No. 462,631 shows a variant of the known art comprising a membrane coupled to a reflective part which may be moved down in front of the end of an optical fibre. The amount of light being reflected back thus depends upon the pressure affecting the membrane.
The solution described in the Swedish patent is relatively compact and may e.g. be used in medical applications, but has a limited sensitivity, as it is not based on interferometry.
DE 40.35.373 describes a Fabry-Perot interferometer positioned at the end of an optical fibre. It is not mentioned how the interferometer is made, but it will most likely come into the abovementioned category of relatively expensive solutions. Also the partially reflective surfaces in this interferometer has limited possibilities for relative movements, which will result in either a limited dynamic range or a limited resolution.
The German patent publication also describes an example of the use of such an interferometer, and a method for calculating the pressure based on the measurements.
Another type of sensors based on optical fibres known in the art uses two optical fibres fastened end to end in a casing. Deformations in this casing may be measured as it affects the distance between the fibre ends. This system is relatively rigid and is not suitable for measuring pressure, but is used for measuring tension.
The object of this invention is thus to provide a miniature pressure probe being inexpensive in production and at the same time maintains the possibilities of the fiberoptic systems for making exact measurements. The pressure probe must be sufficiently small to be used in medical applications, e.g. put into the blood stream of a patient. This obtained using an optic pressure sensor as described above being characterized in that a body is positioned into the cavity, said body having a first at least partially reflective surface, said partially reflective surface, fibre and casing delimiting a chamber, said chamber containing a compressible fluid, and in that at least one of the body and the optical fibre is movably connected to the casing in the axial direction thus to provide a pressure coupling between the chamber and the environment.
This way a simple Fabry-Perot interferometer is obtained in which the external pressure moves a movable body with a partially reflective surface toward or away from a partially reflective end of an optical fibre or light guide, thus affecting the distance between the mirrors in the resonator.
According to an especially preferred embodiment of the invention the first optical fibre is fastened to the casing, while the movable body is a second optical fibre being loosely positioned in the casing. The chamber is sealed using the capillary effect between the second optical fibre and the casing, either by placing a liquid, e.g. silicone oil, at the outer end of the fibre, or possibly second fibre, and let it be drawn into space between them, or possibly the liquid in which the pressure is to be measured may be drawn into the intermediate space. The liquid layer between the body and the casing provides, in addition to sealing the chamber, a lubricating effect and reduces the friction between them. This increases the precision and the speed of the measurements. In addition the sensitivity of the sensor may be adjusted by simply adjusting the distance between the fibres before use by controlling the amount of fluid being positioned in the chamber.